This invention relates to amplifier circuitry particularly but not exclusively intended for use in selective connection to either of two antennas feeding a radio receiver.
Space diversity can reduce the effect of multipath fading in radio receivers. By using two antennas placed an adequate distance apart, a receiver can be made such that it selects the one with the strongest signal, thereby giving an overall improvement in reception. FIG. 1(a) shows the basic concept in which a switch selectively connects a low noise amplifier (LNA) forming part of the receiver to one or other of the two antennas. This arrangement, however, is generally not practical since the switch will have some insertion loss which will degrade the receiver's sensitivity to weak signals. FIG. 1(b) shows the common topology with the switch moved farther down the receiver chain for greater sensitivity. In this case two LNAs are connected between the respective antennas and the switch such that, in effect, the switch selects the output of one or other of the LNA's. A problem with this arrangement is that, as both LNA's are running continuously, it requires twice the current and is more complicated because it requires additional output switch circuitry.
The problem of high current requirement is overcome in a device marketed under the part number TQ9203 by Triquint Semiconductor, Inc. as described in the Triquint data sheet entitled "Low-Current RFIC Downconverter" and dated Apr. 21, 1994. This device is a multifunction RF downconverter in which an LNA section comprises two parallel connected common source FET (field effect transistor) amplifiers each having an input connected to a respective antenna and a common output. A "Select" or "Control" terminal controls a bias circuit connected directly to the gates of the two transistors such that, when a control signal indicative of a stronger reception signal on one antenna is applied, the transistor connected to the one antenna is switched on and the other transistor is switched off and, when the control signal has a value indicative of a stronger signal on the other antenna, the transistors are switched to the opposite state. In this way the stronger antenna signal is conducted to the common output.
One problem with the Triquint device is that it has a relatively high gain variation with temperature. In addition it is designed only for operation in the range 800 to 1000 MHz and would not work at low frequency because of AC coupled inputs.